Field of the Invention
This invention relates to electronic devices and to oscillator circuits and more particularly to generating clock signals for electronic devices.
Description of the Related Art
In general, electronic oscillator circuits are used to generate repetitive oscillating electronic signals for a variety of integrated circuit applications (e.g., local oscillator signals for radio frequency mixers, transmitters for generating carrier waves for radio frequency signal transmission, etc.). Typical clock generator applications can tolerate little frequency error. Accordingly, external crystals may be used to generate precise mixing or carrier clock signals. Crystals are electromechanical devices that may be trimmed to resonate at a particular frequency. Use of a discrete crystal for a crystal oscillator increases printed circuit board area and system cost. The connections between an integrated circuit and the crystal may couple unwanted electromagnetic signals, thereby causing jitter in the resulting clock signal. In addition, the frequency of the clock signal may vary unpredictably due to changes in capacitance on pins coupled to the crystal caused by mechanical stresses on the pins.
Referring to FIG. 1, rather than use a crystal oscillator circuit, a clock generator circuit may use a conventional tank circuit, e.g., tank circuit 102, which is a tuned circuit including inductor 104 coupled to capacitor 106. Charge flows back and forth from the capacitor plates through the inductor so the tuned circuit can store electrical energy oscillating at its resonant frequency. Amplifier circuit 108 compensates for small losses in the tank circuit to sustain oscillation. By supplying a transconductance, −Gm, that is equal and opposite to the tank losses (modeled as Gloss), amplifier 108 is able to sustain oscillation indefinitely at the resonant frequency of the tank circuit and at an amplitude determined by the amplifier. Typically, that amplitude is based on the intrinsic limiting behavior of the amplifier. Accordingly, −Gm should be interpreted as an effective transconductance presented to the tank circuit by the amplifier having an absolute value that monotonically decreases with increasing signal amplitudes for a particular bias condition. That is, the amplitude of signals within the oscillator will increase until the effective transconductance of the amplifier is equal and opposite to the tank circuit losses. In that open-loop approach, the amplifier contributes more loading and power to ensure sufficient excess gain under various environmental (temperature, strain, aging, etc.) and manufacturing (e.g., dielectric thickness, conductor thickness, charge carrier mobility, etc.) conditions to regenerate and sustain oscillation.
For oscillator applications that require low power for a particular performance level, the amplifier is typically biased to reduce or eliminate any excess gain. However, that amplifier bias point may vary as a function of environmental factors (e.g., temperature, strain, aging, etc.), causing the amplitude of the output signal to vary, and therefore, substantially degrade the oscillator performance. Automatic amplitude control techniques may be used to compensate for the effects of those environmental factors. Nevertheless, target performance (e.g., low power consumption for a particular amount of phase noise) may be difficult to achieve using conventional automatic amplitude control techniques. For example, flicker noise (i.e., 1/f noise) is low frequency noise that modulates the frequency of the oscillating signal. Frequency multiplication of the oscillator output signal in some applications (e.g., frequency synthesizers) upconverts the flicker noise and may cause the oscillator to operate outside of a target operating specification. Accordingly, improved oscillating signal generation techniques are desired.